This invention relates to a hydrometallurgical process for the recovery of lead oxide, zinc oxide, electrolytic manganese dioxide (EMD) and optionally precipitated calcium carbonate (PCC) from Pb, Zn, Mn, and optionally Ca, dissolved in aqueous solution or leachate (e.g. as produced by the method described in U.S. Pat. No. 5,523,066) and preferably to the combination of said process and said leaching thereby forming a closed loop system to recover the foregoing products in a manner that reduces environmental impact and economic costs to enable recovery of minerals from deposits that would be too expensive by traditional means. This zero discharge closed loop system can be expanded to also recover with equal advantage other metals dissolved by the leach method e.g. of said ""066 patent. The process incorporates three selected solvent extraction steps in a particular sequence.
PbO (and PbO2 its precursor) is vital for high performance electric batteries for automobiles and electronic appliances, ZnO is useful to manufacture automobile tires, paint, pharmaceuticals and as an additive for both human and animal food, EMD is a main ingredient for alkaline batteries used for electronic equipment and PCC is a principal component of fine paper. There are known methods to produce these products but none are able to do so with the environmental, cost and sustainable development advantages offered by the combination methodology of the present invention.
The traditional methods to derive the said products entail some combination of mining, concentration, smelting and refining to initially extract the basic raw materials i.e. Pb, Zn, Mn and Ca followed by transforming the materials to the final product conforming to industrial specifications. PbO and ZnO usually are made by oxidizing the metal either by ball milling or blowing air through its molten form; MnO2 is recovered usually by electrowinning leachate from the oxidized Mn ore. In most cases, however, without purification the MnO2 product is sub-standard causing explosion of batteries in which it is used; while calcium carbonate preparation may involve calcining limestone that creates considerable greenhouse gas as CO2. The sum of these activities form a tedious series of steps that are expensive, replete with environmental and personal health problems, hazardous risks and the possibility of creating ugly scars to the landscape. The traditional methods entail massive land disturbance from pervasive blasting and excavations in the course of mining that can result in permanent alteration of ground water levels in the vicinities of pits, adits, waste dumps and tailings ponds. Lowering the water table through mining activities adversely affects agriculture and forestry and can expose sulphide minerals to unsaturated hydrological conditions and to rapid weathering that greatly increase the potential for long term production of acid rock drainage (ARD) and toxic metal migration.
Although Pb, and Mn can be selectively recovered electrolytically as oxides directly from multi-component leachate, e.g. produced by the ""066 patent, by suitably controlling the voltage of the electrolytic cell, and Zn can be recovered similarly as a hydroxide thence converted by heating to an oxide, such products are impure because they adsorb other components in the leachate due to the surface reactivity e.g. ion exchange capacity of the products thereby becoming impure requiring cleansing to remove adsorbates to achieve commercial specifications and such cleansing is complicated and expensive. Moreover as the concentration of the Pb, Zn and Mn in the said leachate decreases during electrolytic recovery, such recovery becomes increasingly inefficient reaching a point where it is uneconomic to further recover metal in this fashion.
In U.S. Pat. 4,191,729, Mar. 4, 1980 G. Rinelli, et al, a process for leaching lead and zinc from oxidized ore using ammoniacal (with polycarboxylic acid anion) solutions at pH 11.5-12.5 is described. Selective solvent extraction of lead from the leachate using LIX-64N(trademark) (mixture of hydroxy-aryl-, and -alkyl, -oximes) as active extractant is carried out at pH 9-10. The zinc is precipitated from residual leachate by heating.
In U.S. Pat. 5,523,066, Jun. 4, 1996, R. A. Geisler, et al, a process is described for leaching lead and other metals from sulphidic ores using acetate solutions in the presence of an oxidant. In many cases zinc, manganese and other metals are present and are also leached. Recoveries from leach solution is by known techniques (with disadvantages mentioned above). It would be desirable to selectively separate and recover high purity lead, zinc and manganese from such leach solutions in a more efficient manner.
U.S. Pat. 5,028,403, Jul. 2, 1991, W. A. Rickelton, et al., describes solvent extraction of certain metals from aqueous solutions with monothiophosphinic acids as extractant. Extraction (from sulfuric acid solutions) of zinc and manganese is described, but not lead.
Many of the foregoing problems inherent in traditional recovery of the said products are reduced by the methodology of the present invention because it minimizes or does not create waste piles, toxic discharges, water table alteration, substantial ground disturbance or human exposure to hazardous risk. Any CO2 that is generated from leaching by the ""066 patent methodology may be utilized to form PCC. Recovery of the products as particulates enables more automated control of their production, specifications, packaging and storage. Electrolytic cell design employing gas diffusion electrodes for counter electrodes can decrease power used by up to about 80%. Accordingly the invention has significant potential for being both an economically and environmentally sustainable mineral resource development technology.
We have surprisingly found that Pb, Zn and Mn can be selectively recovered from aqueous solution or leachate each into a separate extractant and a strip liquor forming respective dedicated solutions containing essentially no other metallic impurity. Each liquor not only provides a source enabling a purer product to be recovered electrolytically but moreover permits more of the said Pb, Zn and Mn dissolved in the feed solution or leachate to be economically recovered without incurring increasingly higher power costs as would otherwise be necessary. Such feed solutions may be derived e.g. from sulfide and/or carbonate ores.
A series of hydrometallurgical steps forming a process is described that can be joined as a preferred option with a leach method, e.g. of said ""066 patent, to form a combined closed loop method that is environmentally and economically advantageous compared to existing methods to recover the desired products e.g. PbO, ZnO, EMD and optionally PCC respectively from Pb, Zn, Mn and optionally Ca (+Mg+Ba) that are dissolved usually along with many other elements in such leachates. The final products can be specially tailored for industrial use by controlling the particle size, surface area and morphology of the individual products for particular tasks. All are substantially pure except the PCC that usually includes MgCO3 plus other leachate components which can make it acid tolerant for use in making paper.
The invention includes a process for recovering lead, zinc, and manganese from an aqueous solution containing these metals, comprising the selection of a solvent extractant able to preferentially extract lead in the presence of zinc and manganese, from the solution; providing that the solution is free of interfering metals; extracting the interferant-free solution with selected solvent extractant in an amount sufficient to extract the lead and separating the loaded extractant phase from the solution; selecting solvent extractant able to preferentially extract zinc in the presence of manganese; extracting the lead-free solution with selected solvent extractant in an amount sufficient to extract the zinc and separating the loaded extractant phase from the solution; selecting solvent extractant able to preferentially extract manganese in the presence of any residual metals; extracting the lead- and zinc-free solution with selected solvent extractant in an amount sufficient to extract the manganese and separating the loaded extractant phase from the solution; separately stripping each loaded extractant phase to move lead, zinc and manganese into separate aqueous acid strip solutions; and recovering, in a selected form, lead, zinc and manganese from the corresponding strip solutions.
The invention includes the use, as a preferred option, of selected monothiophosphinic acids as active extractant in each extraction step, as well as the option of using selected alternative extractants for each extraction step as described below.
The invention also includes the option of recovering lead from its strip solution at the anode of an electrolytic cell, and concurrently producing peroxide at the opposite electrode for use in leachate formation.
The invention includes the option of using CO2 released during leaching to precipitate alkaline earth metal carbonates downstream of solvent extraction.
The invention includes a process for recovering lead, zinc, manganese and calcium from aqueous acetate solutions containing these but free of copper, iron, cadmium and cobalt, comprising:
(a) extracting the aqueous solution at a pH of from 0.0 to 0.5 with an organic phase comprising substituted monothiophosphinic acid to extract lead;
(b) extracting the lead-free aqueous solution at a pH of from 1.0 to 1.2 with an organic phase comprising at least one of substituted monothiophosphinic acids, disubstituted-phosphoric and -phosphorothioic acids, and C26H34SN2O2 known as LIX 34(trademark) to extract zinc;
(c) extracting the lead- and zinc-free aqueous solution at a pH of from 3.0 to 3.5 with an organic phase comprising at least one of substituted monothiophosphinic acids, disubstituted phosphinic acids, and disubstituted-phosphoric and -phosphorothioic acids to extract manganese;
(d) stripping each of the loaded organic phases with an aqueous acid solution comprising one acid from the group consisting of acetic, nitric, hydrochloric and sulphuric except that sulphuric may be selected only for Mn stripping;
(e) electrowinning lead, zinc and manganese from their respective strip solutions and recycling the depleted strip acids to step (d);
(f) treating the remaining aqueous solution containing calcium, as acetate or hydroxide, with carbon dioxide to precipitate calcium carbonate;
(g) recovering and recycling acetic acid to form starting solution; and
(h) recovering lead, zinc and manganese as oxides and a precipitate comprising calcium carbonate.